Generally, fine pattern formation is carried out by photolithography in manufacturing processes of a semiconductor device. A number of transfer masks called photomasks are normally used for this fine pattern formation. The transfer mask comprises generally a transparent glass substrate having thereon a fine pattern made of a metal thin film or the like. The photolithography is used also in the manufacture of the transfer mask.
In the manufacture of a transfer mask by photolithography, use is made of a mask blank having a thin film (e.g. a light-shielding film or the like) for forming a transfer pattern (mask pattern) on a transparent substrate such as a glass substrate. The manufacture of the transfer mask using the mask blank comprises a writing process of writing a required pattern on a resist film formed on the mask blank, a developing process of, after the writing, developing the resist film to form a required resist pattern, an etching process of etching the thin film using this resist pattern as a mask, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after writing the required pattern on the resist film formed on the mask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using this resist pattern as a mask, an exposed portion of the thin film, where the resist pattern is not formed, is removed by dry etching or wet etching, thereby forming a required mask pattern on the transparent substrate. In this manner, the transfer mask is completed.
As a type of transfer mask, a phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material on a transparent substrate.
In recent years, with higher integration of semiconductor devices, patterns finer than the transfer limit of the photolithography using the conventional ultraviolet light have been required in the semiconductor industry. In order to enable the formation of such fine patterns, the EUV lithography being an exposure technique using extreme ultraviolet (Extreme Ultra Violet: hereinafter referred to as “EUV”) light is expected to be promising. Herein, the EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 to 100 nm. A reflective mask has been proposed as a mask for use in the EUV lithography. In the reflective mask, a multilayer reflective film for reflecting EUV light as exposure light is formed on a substrate and an absorber film for absorbing the EUV light is formed in a pattern on the multilayer reflective film.
With the increasing demand for miniaturization in the lithography process as described above, problems in the lithography process are becoming remarkable. One of them is a problem about defect information of a substrate for a mask blank or the like for use in the lithography process.
Conventionally, taking the center of a substrate as the origin (0,0), the existing position of a defect of the substrate is specified by the distance from the origin (0,0) in mask blank inspection or the like. As a consequence, the position accuracy is low and there is variation in detection among apparatuses and thus, when patterning a pattern-formation thin film while avoiding the defect at the time of pattern writing, it is difficult to avoid it on the order of μm. Therefore, the defect is avoided by changing the direction of pattern transfer or roughly shifting the pattern transfer position on the order of mm.
Under these circumstances, for the purpose of enhancing the inspection accuracy of a defect position, there has been a proposal, for example, to form a fiducial mark in a substrate for a mask blank and to specify a position of a defect using the fiducial mark as a reference position.
Patent Document 1 discloses that, in order to accurately specify a position of a minute defect having a sphere-equivalent diameter of about 30 nm, at least three marks each having a sphere-equivalent diameter of 30 to 100 nm are formed in a film-forming surface of a substrate for a reflective mask blank for EUV lithography.